United States

Fused silica with its outstanding chemical and thermal stability as well as excellent optical properties is widely used in the optics, microsensors and medical technologies. However, its robustness and brittleness pose challenges on processing it with conventional methods. In turn, ultrashort pulsed lasers with their unique properties offer the possibility for its gentle machining. Recent developments in laser and optics technology have led to the availability of frequency tripled ultrashort pulsed laser systems emitting in the ultraviolet spectral region with high pulse energy and average power, while exhibiting high beam quality and durability, motivating their application in micromachining of fused silica.&lt;br&gt;
We report on ultraviolet ultrashort pulsed laser (UV) ablation of fused silica and compare the achievable machining results to the fundamental emission wavelength in the infrared (IR). The UV reveals a stable ablation efficiency at increasing fluences, while IR exhibits a decreasing trend. In addition, a significant improvement of surface quality is found by using UV laser in low and middle fluence regime as compared to IR. Specifically, a low surface roughness is found for UV with keeping a similar ablation efficiency of both wavelengths at middle fluence (about 10 J/cm&lt;sup&gt;2&lt;/sup&gt;).&lt;br&gt;
Identifying and taking advantage of the improved ablation results by using UV-ultrashort pulsed laser, we finally present exemplifying and sophisticated application examples of UV ultrashort pulsed laser ablation of fused silica.

ICALEO 2023

UV-Ultrashort Pulsed Laser Ablation of Fused Silica

micromachining

ultrashort pulsed laser

uv-laser

fused silica

Fused silica with its outstanding chemical and thermal stability as well as excellent optical properties is widely used in the optics, microsensors and medical technologies. However, its robustness and brittleness pose challenges on processing it with conventional methods. In turn, ultrashort pulsed lasers with their unique properties offer the possibility for its gentle machining. Recent developments in laser and optics technology have led to the availability of frequency tripled ultrashort pulsed laser systems emitting in the ultraviolet spectral region with high pulse energy and average power, while exhibiting high beam quality and durability, motivating their application in micromachining of fused silica. 
We report on ultraviolet ultrashort pulsed laser (UV) ablation of fused silica and compare the achievable machining results to the fundamental emission wavelength in the infrared (IR). The UV reveals a stable ablation efficiency at increasing fluences, while IR exhibits a decreasing trend. In addition, a significant improvement of surface quality is found by using UV laser in low and middle fluence regime as compared to IR. Specifically, a low surface roughness is found for UV with keeping a similar ablation efficiency of both wavelengths at middle fluence (about 10 J/cm2). 
Identifying and taking advantage of the improved ablation results by using UV-ultrashort pulsed laser, we finally present exemplifying and sophisticated application examples of UV ultrashort pulsed laser ablation of fused silica.

poster

**General Chair Welcome**

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Welcome to ICALEO® 2023!
	
We were excited to host the world’s leading international conference on Lasers and Electro-Optics in Chicago. The Laser has been shown to have the potential to revolutionize solutions for major global challenges like energy usage and supply, global warming, and CO2 footprint. Even the fusion energy breakthrough is based on laser technology. The conference, with peer-reviewed scientific and technical presentations, focused on the newest results in Micro Applications, Macro Applications, Additive Manufacturing, Beam Shaping and Frontiers in Laser Applications. Additionally, the Battery-Manufacturing Track gave an overview of the process combined with the newest advances in laser application results. 

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**Klaus Löffler **  
ICALEO General Chair   
Managing Director, Precitec GmbH

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ICALEO® brings together the brightest minds, seasoned professionals, and pioneering researchers from around the world. With a laser-focused agenda, ICALEO offers an exceptional platform to delve into the cutting-edge advancements, diverse applications, and transformative research that define lasers and electro-optics.

Surface tortuosity is a measure of the complexity of the surface of a material, which is typically expressed as the ratio of the length of the surface over the shortest distance between two points on the surface. This characteristic has been considered beneficial for coating/steel stability of painted parts exposed to corrosive and humid conditions. Laser surface texturing process can modify the surface tortuosity by producing different structures on the steel surface. However, due to the versatility of the process, structures with different sizes and magnitudes can be fabricated on the surface giving a wide range of possibilities. In this context, this study aims to determine the influence of the V-shaped grooves dimensions into the resistance against corrosion creep of an organic coating applied on textured surfaces. Comparative surface tortuosity was obtained with deferent grooves dimensions at equal relation aspect and textured areas. Aspect ratio of one and sizes of 50, 100 and 200 µm were fabricated on AISI-A36 carbon steel surface. Distance between adjacent grooves was varied to achieve different textured areas in the range of 10 % to 60 %. Surface roughness Rz and surface tortuosity was characterized. Coating performance was evaluated through an accelerated corrosion test based on ISO 12944-9. Results show that lower coating delamination was obtained with V-shaped grooves with 100 µm and a textured area of 40 %. Delamination of the coating was diminished by the increase of surface tortuosity only if mechanical interlock/hooking of the coating is enhanced.

Evaluation of Surface Tortuosity on the Corrosion Resistance of Organic Coatings Using Laser Textured Surfaces

Copper is widely used in high heat flux and electrical applications due to the excellent electrical and thermal conductivity properties. Alloying elements such as chromium or nickel are added to strengthen the material especially for higher temperatures. Cu4Cr2Nb or GRCop-42 is a dispersion strengthened alloy developed by NASA for high temperature applications with high thermal and mechanical loads such as rocket engines. Additive manufacturing enables applications with complex functionalised geometries and is especially promising in the aerospace industry. In this contribution, a parametric study was carried out for GRCop-42 and the AM process laser powder bed fusion using a green laser source for two layer thicknesses of 30 and 60 μm. The density, electrical conductivity, macro hardness, microstructure and static mechanical properties were analysed. Various heat treatments in the temperature range of 400 °C and 1000 °C and time range of 30 min to 4 h were tested to increase electrical conductivity and hardness. For both layer thicknesses, dense parameter sets could be obtained with resulting relative densities above 99.8 %. The hardness and electrical conductivity could be tailored depending on the heat treatment in the range of 103 to 219 HV2 and 24 to 88 % International Annealed Copper standard. The highest ultimate tensile strength obtained was 493 MPa. An annealing temperature of 700 °C for 30 min has shown the best combination of room temperature properties such as electrical conductivity of 83.76 %IACS, UTS of 481 MPa, elongation at break at 24 % and hardness of 125 HV2.

Process Development for Laser Powder Bed Fusion of Grcop-42 Using a 515 NM Laser Source

There has been strong interest in laser direct lithography(LDL) due to flexibility of patterning based on software. However, the conventional LDL has a problem that is too slow compared to mask lithography because it depends on the scanning velocity of laser beam or stages, which causes a decrease in productivity, making it difficult to apply to manufacturing. To overcome these drawbacks, we implemented a resonant mirror based LDL, which is different from the conventional laser scanning method based on a stepper motor. The laser source of the instrument is a laser diode(LD) which has 405 nm wavelength, and the optics are designed to have a beam spot size of less than 20um in the focal plane. We applied this feature to laser scan system and have set 800Hz frequency. It can create a pattern in 1.25us for a line of about 10 mm. Compared to the conventional LDL system using the laser scanning method, the resonant mirror based LDL system has the advantages of being over 80,000 times faster in processing time. The designed LDL system capable of high-speed scanning with uniform physical characteristics is expected to meet the demand for small quantity production of diverse items as well as low operation costs.

Resonant Mirror Based Laser Direct Lithography

Laser welding using high-power laser forms a keyhole inside a molten pool to obtain a deep penetration. However, a spatter which is the scattering of a portion of the molten pool occurs causing welding defects. Therefore, it is necessary to elucidate the mechanism of spatter generation. In our previous study, it has been found the spatter generation is due to the variability of the keyhole. However, it is difficult to understand the spatter generation from keyhole observation only. In addition to the keyhole variability, real-time observation of molten pool dynamics would be understood the spatter generation mechanism. 
In this study, the longitudinal cross-section of the molten pool was directly observed using the glass transmission method to measure the temporal variation of the molten pool area in keyhole welding. A bead-on-plate welding test was conducted for the specimen with a 16 kW disk laser when the SS304 specimen butted against a glass plate was set in a vacuum chamber. The vacuum chamber was evacuated at the pressure of 10 Pa and then filled with Ar gas to the desired pressure. As the results, the average molten pool area was 17 mm2&nbsp;at 105 Pa, which generate a large amount of spatter, while, it was 29 mm2 at 10 Pa, where the spatter generation was small. Furthermore, the standard deviation of the molten pool area was 0.9 mm2 at 105&nbsp;Pa, and 0.36 mm2 at 10 Pa, it was indicated a large molten pool time variation was influenced to generate the spatter.

Elucidation of Spatter Generation Mechanism in Keyhole Welding of Stainless Steel With 16 kW Disk Laser by In-Situ Observation of Welding Dynamics

Besides applications like layering functional surfaces, laser cladding can be used for additive manufacturing and repair welding purposes called directed energy deposition (DED). DED is a more and more used technology in industry and research today. It fills the gap between high-resolution powder bed selective laser melting and high output wired arc additive manufacturing processes. 
The temperature-time-regime is important to be managed in the whole process. With increasing melt pool sizes according to heat accumulation, distortions and residual stresses get out of control. Hence, the part could be cooled between each layer. The temperature could be measured in order to derive the cooling time. However, if the part geometry is known and repeated in an industrial process, the cooling time will always remain the same. Modelling the process with a Finite Element Analysis (FEA) can also help to predict the correct cooling time. In order to achieve suitable results of the FEA Model, the Model has to be calibrated. Moreover, calculation times of FEA can be high according to the complexity and dimensions. 
In powder based additive manufacturing process FEA it is quite common to work with replacement heat sources. These Approaches cannot be transferred to DED because of the process conditions. However, some simplification in the model might lead to sufficient results. In this study we have investigated a process with 15 thermocouples and compared the results with a FEA model. The influence of the process efficiency, heat radiation and thermo-physical-properties in the FEA result is shown.

Finite Element Analysis and Calibration of a Directed Energy Deposition Process by Thermocouples

Cobalt based tungsten carbide composite material [WC-Co] is industrial applied for coating on tip of cutting tools and molds due to its high hardness and wear resistance. However, it is difficult to coat composite materials while maintaining the properties of the powder because of the different melting points of the metals. Thus, we pay attention to a Laser metal deposition (LMD) method. The LMD is one of additive manufacturing technologies in which the material powder is fed, and laser irradiated simultaneously at the processing point to melt and solidify the material powder to form layers, which can add new functions to material surface of any shape. However, conventional LMD has some problems to heat the powder ununiformly in flight, which a molten pool is required on the substrate to weld with powder and substrate, it was caused significantly thermal distortion and dilution. Thus, we have developed a multi-beam LMD system with two high intensity 200W blue diode lasers. The multi beam LMD can heat uniformly the powder in flight resulting in lower thermal distortion and dilution. Especially, using a blue laser with a shorter wavelength than the IR laser used in conventional LMD can improve the light absorption rate for WC-Co power and the steel substrate, to save an energy compared with the conventional LMD. In this study, we try to form a WC-Co coating on a steel substrate surface using the multi-beam LMD system equipped with two blue diode lasers.

Experimental Evaluation of WC-Co Alloy Layer Formation Process by Multi Beam Type Laser Metal Deposition With Blue Diode Lasers

Controlling the wetting characteristics of fluids (oil and water) on surfaces is highly desirable for a variety of industrial processes, such as painting, bonding, and lubrication applications. This can be achieved by altering the surface chemistry and/or topography of a material, which can be accomplished through a variety of techniques, including plasma treatment, chemical modification, surface coatings as well as microprocessing methods such as direct laser writing and direct laser interference patterning. 
This study summarizes recent developments in the field of functionalization of polymer surfaces, including polyethylene terephthalate (PET), polycarbonate (PC) and polytetrafluoroethylene (PTFE) using hot-embossing methods as well as direct laser fabrication approaches. 
The influence of different hot embossing parameters on the resulting texture topography was investigated. In case of the oil wettability tests (on PET), a method to characterize its spreading behavior was developed, allowing a comparison between the produced surface textures. In particular, a superoleophilicity state was reached, in which the contact angle approached zero. Furthermore, hierarchical structured surfaces showed fast wetting rates up to 1.4 mm²/s. 
Regarding water-wettability, the microstructured PET surfaces showed an increased hydrophobic state rising from 76° on the flat reference up to 120°. In case of PC, similar results are obtained, reaching 140°, whereas on PTFE surfaces a super-hydrophobic state was achieved, characterized by water contact angles over 150° and a very small hysteresis (&lt; 5°).

Water and Oil Wettability Customization by Tailoring Micro-Structured Polymers

Globally, the supply of electric vehicles is rapidly increasing due to the regulation of CO2 emission. Laser welding is essential for core parts of electric vehicles, and welding of copper (Cu) material accounts for a large proportion. Unstable welding quality can cause fire and threaten the life of the operator. Quality is urgently needed. In-depth research on Cu sheet laser welding is needed. Laser welding for electric vehicle battery uses a lot of copper. However, IR laser is difficult to achieve stable melting and quality control due to the high reflectivity of copper, and the beam penetrates the battery, causing cell damage and fire. In this study, core parts such as cylindrical batteries of electric vehicles are welded using a green laser for stable fusion and quality. In this study, a 2 kW green laser welding system and a 6-axis industrial robot were used. Green laser welding process technology is applied using nickel-coated 0.5 mm copper on the top and 0.5 mm mild steel on the bottom. Optical microscope analysis was used to examine the penetration after green laser welding.&nbsp; The laser welding microstructural modification was investigated using scanning electron microscope(SEM) and energy disperse X-ray spectrometer(EDS). After green laser welding, the result is measured using shear stress test. Moerever, Using Laser welding monitoring system(LWM) the causes of welding defects were identified and the process conditions for welding quality deterioration were matched.

A Study on Green Laser Welding Characteristics of Core Materials for Electric Vehicles

Enhancing the durability of molds, jigs, and tools is crucial for the industry, and one approach to achieve this is by forming a metallic layer with high hardness on their surfaces. Metallic layers with high hardness can be formed through laser metal deposition (LMD), which is one of the additive manufacturing processes, using cemented carbide powder. However, crack initiation typically occurs inside cemented carbide layers formed by the LMD. Therefore, achieving a cladding process for cemented carbide layers without cracks is desired for practical applications. In this study, effects of WC ratios in WC-Co cemented carbide granulated powder on formed bead size and crack initiation during the LMD processing were investigated. The number of cracks generated during the LMD processing was evaluated using an acoustic emission (AE) technique. The number of burst-type AE signals generated was counted as the number of cracks. Seven types of WC-Co cemented carbide granulated powders with WC ratios ranging from 30.5mass% to 92mass% were prepared. Beads were formed using each powder through the LMD, with AE signals being measured. In the case of a WC ratio of 42.9mass% or less, no crack was observed. On the other hand, cracks were observed when the WC ratio was 53.9mass% or greater, and the number of cracks increased with an increase in the WC ratio.

Effects of WC Ratios on Bead Size and Crack Initiation in Forming WC-Co Cemented Carbide by Laser Metal Deposition

Inconel has been used in many industrial components such as a blade of gas turbine because of having its high creep strength and high corrosion resistance. The gas turbine for next generation requires more complex shape to improve the performance. A selective laser melting (SLM) method is one of additive manufacturing (AM) technologies, which is suitable for making metal objects with complex shape rapidly. The AM for metals is a fabrication method that enables direct modeling of three-dimensional (3D) products from a metal powder based on 3D-CAD data. The AM method using a laser as a heat source is classified as selective laser melting (SLM) technology. In SLM, the powder absorbs the laser energy to heat. &nbsp;Then the powder melts and solidifies to form a single layer. Generally, IR lasers with a wavelength of around 1 um are employed in SLM process. Recently, a high-power blue diode laser has been developed.&nbsp; Blue and IR lasers have different light absorption rates for Inconel, which may affect samples fabricated with SLM. 
&nbsp;In this study, 3D fabrication of Inconel was demonstrated by the blue and IR laser, respectively to compare with the process and mechanical property. As a result, high efficiency and fine object was obtained with blue laser.

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UV-Ultrashort Pulsed Laser Ablation of Fused Silica

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Evaluation of Surface Tortuosity on the Corrosion Resistance of Organic Coatings Using Laser Textured Surfaces